In recent years, worldwide demand for wireless cellular communications has increased dramatically. Mobile telephones manufactured to meet this burgeoning demand must adhere to standards such as the Global System for Mobile Communications (GSM) standard. Another standard, the Digital Cellular System (DCS) standard, is based on GSM, but is directed towards higher cell density and lower power. A third standard, Personal Communications Services (PCS), is a “catch all” for many digital cellular systems, including GSM, operating in North America. These standards all require precise output power control over a large dynamic range in order to prevent a transmitter located in one cell from interfering with the reception of transmissions from other transmitters in neighboring cells.
One of the primary goals in designing mobile telephones is to reduce power consumption in order to increase battery-life. In large part, the power consumption of a mobile telephone is dependent upon the RF efficiency of a power amplifier in the transmit chain that operates to amplify a transmit signal prior sending the signal to the antenna for transmission. As described in U.S. Pat. No. 6,701,138, POWER AMPLIFIER CONTROL, issued Mar. 2, 2004, which is assigned to RF Micro Devices, Inc. of 7628 Thorndike Road, Greensboro, N.C. 27409 and is hereby incorporated by reference in its entirety, the efficiency of a multi-stage power amplifier may be increased by reducing a supply voltage provided to one or more output amplifier stages of the multi-stage power amplifier as the output power is reduced such that the output stages of the power amplifier continue to operate in saturation.
As described in U.S. Pat. No. 6,701,138, the supply voltage provided to the power amplifier may be controlled using a voltage regulator. The voltage regulator includes a series pass element, such as a field effect transistor (FET) coupled in series between a source voltage, such as a battery, and an input terminal of the power amplifier. The voltage regulator is controlled to control the supply voltage provided to the input terminal of the power amplifier. However, as output power is reduced, the supply voltage is also reduced. In doing so, the voltage drop across the series pass element is increased, and a larger portion of the power supplied from the battery is dissipated as heat.
Accordingly, there remains a need for a system that further improves the efficiency of a power amplifier.